Introduction
Conjugated linoleic acid (CLA) is a mixture of positional and geometric isomers of linoleic acid, which is found preferentially in dairy products and beef. Thus, dairy products are one of the major dietary sources of CLA. Based on laboratory animal research, CLA is has been found to be anticarcinogenic (especially for breast cancer), it may work as an antioxidant and play beneficial roles in preventing coronary heart disease, may enhance immune function, and may also affect growth and body composition by diminishing fat deposition while promoting lean mass growth.
It was noted long ago (Booth et al., 1935; Reil, 1963) and substantiated by recent research data (Dhiman et al., 1999; Kelly et al., 1998) that cows grazing pasture had higher CLA concentration in milk fat than non-grazing cows. The predominant fatty acid in pasture forage is linolenic acid (18:3 n-3). At present, there is no evidence that linolenic acid can be directly converted into CLA in the rumen during the process of biohydrogenation. It is most likely that linolenic acid is first hydrogenated into vaccenic acid in the rumen and vaccenic acid is then desaturated into CLA postruminally. However, several other factors besides fatty acid content of fresh forage are being investigated to help explain the increase concentration of CLA in milk when cows are grazing pasture.
This project was conducted to provide a reference point for CLA concentration in milk from bulk tanks from herds that primarily graze pasture and are fed organically certified feeds.